Fuel cells are in principle gas-operated batteries in which the energy obtained from the reaction of a fuel gas stream comprising hydrogen and oxygen is converted directly into electrical energy. The present invention describes the preparation of catalysts for preparation of the fuel gas stream for use in fuel cells, in particular for PEM (polymer electrode membrane) fuel cells. This type of fuel cell is becoming increasingly important, due to its high energy density and robust structure, for use in the vehicle industry, i.e. for providing electro-traction in motor vehicles.
The advantages of a vehicle powered by fuel cells are the very low emissions and the high degree of efficiency of the total system compared with conventional internal combustion engines. When hydrogen is the major component in the fuel gas, the primary emission product of the conversion in the fuel cell is water. The water is produced on the cathode side of the fuel cell. The vehicle is then a so-called ZEV (zero emission vehicle). The use of hydrogen in fuel cells requires that hydrogen be available on the anode side of the fuel cell membrane to actually generate power. The source of the hydrogen can be stationary or mobile. Stationary sources of hydrogen will require a distribution and dispensing system like motor gasoline. Mobile sources for hydrogen will include on-board hydrogen generators for the conversion of hydrocarbon fuels to hydrogen. However, hydrogen presents many handling and distribution problems which will not be resolved before the fuel cell powered vehicles reach the market. The infrastructure for the widespread distribution of hydrogen is still too expensive at the moment and there are other problems with the storage and refueling of vehicles. For this reason, the alternative, producing hydrogen directly on board the vehicle by reforming hydrocarbon fuels or oxygenated fuels is growing in importance. For example, methanol can be stored in a fuel tank of the vehicle and on demand converted by a steam reforming process at 200.degree. to 300.degree. C. to a hydrogen-rich fuel gas with carbon dioxide and carbon monoxide as secondary constituents. After converting the carbon monoxide by a shift reaction, preferential oxidation (prefox) or another purification process, this fuel gas, or reformate gas is supplied directly to the anode side of the PEM fuel cell. Theoretically, the reformate gas consists of 75 vol-% hydrogen and 25 vol-% carbon dioxide. In practice, however, the reformate gas also will contain nitrogen, oxygen and, depending on the degree of purity, varying amounts of carbon monoxide (up to 1 vol-%).
The PEM fuel cell comprises layers of catalyst comprising platinum and platinum alloys on the anode and cathode sides of PEM fuel cells. These catalyst layers consist of fine, noble metal particles which are deposited onto a conductive support material (generally carbon black or graphite). The concentration of noble metal is between 10 and 40 wt-% and the proportion of conductive support material is thus between 60 and 90 wt-%. The crystallite size of the particles, determined by X-ray diffraction (XRD), is about 2 to 10 nm. Traditional platinum catalysts are very sensitive to poisoning by carbon monoxide; therefore the CO content of the fuel gas must be lowered to &lt;100 ppm in order to prevent power loss in the fuel cells resulting from poisoning of the anode catalyst. Because the PEM fuel cell operates at a relatively low operating temperature of between 70.degree. and about 100.degree. C., the catalyst is especially sensitive to CO poisoning.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, autothermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream. A paper titled "Will Developing Countries Spur Fuel Cell Surge?" by Rajinder Singh, which appeared in the March 1999 issue of Chemical Engineering Progress, page 59-66, presents a discussion of the developments of the fuel cell and methods for producing hydrogen for use with fuel cells and highlights one hybrid process which combines partial oxidation and steam reforming in a single reaction zone as disclosed in U.S. Pat. No. 4,522,894 which is hereby incorporated by reference.
U.S. Pat. No. 5,922,487 discloses an anode electrocatalyst for a fuel cell which depresses the poisoning of the noble metal fuel cell membrane. The anode electrocatalyst comprises an alloy essentially consisting of at least one of tin, germanium, and molybdenum, and one or more noble metals selected from platinum, palladium, and ruthenium.
U.S. Pat. No. 6,007,934 is concerned with the preparation of supported catalysts based on platinum and ruthenium disposed on the anode side of a PEM fuel cell which have a high resistance to poisoning by carbon monoxide. Carbon monoxide concentrations of more than 100 ppm in the reformate gas should be possible to employ in the fuel gas passed to the fuel cell without a noticeable drop in performance of the PEM fuel cell.
U.S. Pat. No. 6,010,675 discloses a method and apparatus for removing carbon monoxide from a fuel gas prior to use of the fuel gas in a fuel cell for the production of electric power. Catalysts for purifying hydrogen by selective oxidation of carbon monoxide using alumina supported platinum are disclosed in an article entitled "Purifying Hydrogen by . . . Selective Oxidation of Carbon Monoxide" by Marion L. Brown, Jr. et al, Industrial and Engineering Chemistry, Vol. 52, No. 10, October 1960, pp. 841-844. U.S. Pat. No. 6,010,675 discloses the problem of using a conventional preferential oxidation catalyst system in a hydrogen generator or fuel processor for producing a fuel gas stream for use in a fuel cell. The above mentioned article at page 842-3 indicated that the selective removal of carbon monoxide was feasible only within a certain temperature zone for all known selective oxidation catalysts with or without variation of the oxygen concentration, below which the oxygen reaction falls off. The critical temperature range for the effective preferential oxidation was identified as being above 130.degree. C. (266.degree. F.) and below 160.degree. C. (320.degree. F.). U.S. Pat. No. 6,010,675 and the above mentioned article are hereby incorporated by reference. The article stated that this narrow range of selectivity applied to a wide range of precious metal catalysts supported on aluminum oxide.
An article entitled "Advanced PEFC Development For Fuel Cell Powered Vehicles", by Shigeyuki Kawatsu, published in the Journal of Power Sources, Volume 71 (1998), pages 150-155, discloses that a ruthenium catalyst on alumina was found to be useful for reducing the carbon monoxide concentrations of reformed gas from methanol reforming over a wider operating temperature range than platinum based oxidation catalysts. Significant carbon monoxide conversion activity between about 100.degree. and about 160.degree. C. was disclosed.
European publication No. EP 0955351A1 discloses a CO-selective oxidation catalyst having metals including platinum and ruthenium disposed on an alumina carrier. The catalyst preparations included ruthenium metals on alumina pellets with ruthenium metal loadings up to 1.0 wt-%. EP 0955351A1 discloses that the active temperature range for ruthenium was about 160.degree. to 180.degree. C., and only when platinum was either alloyed with the ruthenium or when platinum was included on the alumina carrier was a desired active temperature below 160.degree. C. achieved.
In order to achieve a balance between the reforming reaction zone and the high and low temperature water gas shift reaction zones of fuel processors, others have attempted to dispose these reaction zone in intimate thermal contact to minimize overall energy use. The addition of a preferential or selective oxidation zone to such an integrated system wherein the preferential oxidation catalyst requires effective operating conditions above the outlet conditions of the low temperature water gas shift reaction and above the temperature of the fuel cell operation creates a difficult engineering problem. On the reaction side, the increased temperature may result in hydrogen loss, and on the engineering side, heating the effluent form the water gas shift reaction zone to the favorable temperature range of the selective oxidation reaction and then cooling the selective oxidation effluent requires increased mechanical complexity, and increased equipment cost.
An object of the present invention is to provide preferential oxidation catalysts which have an improved conversion of carbon monoxide. It is an objective of the present invention to provide a preferential oxidation catalyst which operates effectively at conditions which are more favorable in reducing the carbon monoxide concentration in the fuel gas in fuel cell systems. It is an objective of the present invention to provide and, in particular, to achieve effluent concentrations of carbon monoxide of less than about 50 ppm-vol. Another object of the present invention is to provide stable catalysts suitable for the selective conversion of carbon monoxide while maintaining a reasonably high selectivity to the production of carbon dioxide without regeneration.